Silicon carbide is one of the core semiconductor materials of the third generation in our country. As compared with Si and GaAs, silicon carbide has a good number of advantages, such as wide band gap, high thermal conductivity, high electron saturation drift velocity, good chemical stability, etc., which make it very suitable for use in manufacturing electronic devices compatible with high temperature, high frequency, anti-radiation, large power and high density integration. Silicon carbide has gained high attention in various countries owing to its great importance.
Silicon carbide is mainly grown using seeded vapor transport technology (PVT), according to which highly pure (electronic grade) silicon carbide powder is exposed to temperatures of higher than 2000° C. with a temperature gradient along the seed crystals of silicon carbide, so that vapor transport of the Si and C components occurs from the powder to the seed crystals, and oriented growth of silicon carbide crystals on the seed crystals is thus effected. Despite all the advantages of silicon carbide, it has not been used widely for a long time due to its special preparation method in which growth of perfect crystals is difficult. The main challenges faced by the silicon carbide growth technology include enhancement of crystal purity and precise control of impurities, so as to lower the lattice defect density during crystal growth to a large extent. As the predominant component of air, nitrogen serves as a donor impurity for providing electrons to silicon carbide, and may have significant influence on the resistivity of the material even at a concentration of 1 ppm. Thus, it is yet quite difficult to make silicon carbide have an impurity concentration of 1013˜1014 cm−3as in the long base region of a high-voltage silicon thyristor. To date, it is hard to prepare an n-type 6H-silicon carbide crystal with an effective impurity concentration of lower than 1016 cm−3 by sublimation method.
The apparatuses for growing silicon carbide crystals in the prior art generally use a single-chamber structure. For example, U.S. Pat. No. 6,200,917 discloses a vacuum growth chamber which is made from quartz glass and composed of sealing ring, bottom flange, quartz tube and top cap, wherein an induction coil is disposed outside of the quartz glass. For another example, in Chinese Patent Application CN200310113521.X, a metal cavity, in which an induction coil is placed, is used as a single vacuum growth chamber. Since the vacuum growth chamber has to be opened each time for charging the starting material before the crystal growth and removing the crystals after the crystal growth, the vacuum growth chamber contacts atmosphere and is thus extremely susceptible to atmospheric contamination and nitrogen adsorption. Therefore, it is very difficult to achieve crystal growth of highly pure semi-insulating silicon carbide and crystal growth of silicon carbide under precise impurity control.
In summary, the apparatuses used in the prior art for growing silicon carbide crystals can hardly achieve crystal growth of highly pure semi-insulating silicon carbide and crystal growth of silicon carbide under precise impurity control, and thus can not meet the demand of developing power devices on silicon carbide crystals.